IETF MANET Working Group Sung-Ju Lee
INTERNET-DRAFT William Su
Expiration: July 2000 Mario Gerla
University of California, Los Angeles
January 2000
On-Demand Multicast Routing Protocol (ODMRP) for Ad Hoc Networks
<draft-ietf-manet-odmrp-02.txt>
Status of This Memo
This document is an Internet-Draft and is NOT offered in accordance
with Section 10 of RFC2026, and the author does not provide the IETF
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Abstract
On-Demand Multicast Routing Protocol (ODMRP) is a multicast routing
protocol designed for ad hoc networks with mobile hosts. ODMRP is
a mesh-based, rather than a conventional tree-based, multicast
scheme and uses a forwarding group concept (only a subset of nodes
forwards the multicast packets via scoped flooding). It applies
on-demand procedures to dynamically build routes and maintain
multicast group membership. ODMRP is well suited for ad hoc
wireless networks with mobile hosts where bandwidth is limited,
topology changes frequently and rapidly, and power is constrained.
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Contents
Status of This Memo 1
Abstract 1
1. Introduction 4
2. Terminology 5
2.1. General Terms . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Specification Language . . . . . . . . . . . . . . . . . 5
3. Protocol Overview 6
3.1. Multicast Route and Mesh Creation . . . . . . . . . . . . 6
3.2. Reliability . . . . . . . . . . . . . . . . . . . . . . . 8
3.3. Soft State . . . . . . . . . . . . . . . . . . . . . . . 10
3.4. Selection of Timer Values . . . . . . . . . . . . . . . . 10
3.5. Unicast Capability . . . . . . . . . . . . . . . . . . . 10
3.6. Contents of Tables . . . . . . . . . . . . . . . . . . . 11
3.6.1. Routing Table . . . . . . . . . . . . . . . . . . 11
3.6.2. Forwarding Group Table . . . . . . . . . . . . . 11
3.6.3. Message Cache . . . . . . . . . . . . . . . . . . 11
3.7. Mobility Prediction 12
3.7.1. Adapting the Refresh Interval via
Mobility Prediction . . . . . . . . . . . . . . . 12
3.7.2. Route Selection Criteria . . . . . . . . . . . . 14
3.7.3. Alternative Method of Prediction . . . . . . . . 14
4. Packet and Table Formats 15
4.1. Join Query Packet Header . . . . . . . . . . . . . . . 15
4.2. Join Reply Packet . . . . . . . . . . . . . . . . . . . 17
5. Operation 19
5.1. Forwarding Group Setup . . . . . . . . . . . . . . . . . 19
5.1.1. Originating a Join Query . . . . . . . . . . . . 19
5.1.2. Processing a Join Query . . . . . . . . . . . . . 19
5.1.3. Processing a Join Query When GPS is Used . . . . 20
5.1.4. Originating a Join Reply . . . . . . . . . . . . 21
5.1.5. Processing a Join Reply . . . . . . . . . . . . . 21
5.1.6. Processing a Join Reply When GPS is Used . . . . 21
5.2. Handling a Multicast Data Packet . . . . . . . . . . . . 22
6. Simulation and Implementation Status 23
7. Protocol Applicability 24
7.1. Networking Context . . . . . . . . . . . . . . . . . . . . . 24
7.2. Protocol Characteristics and Mechanisms . . . . . . . . . . 24
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Acknowledgments 26
References 26
Chair's Address 29
Authors' Addresses 29
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1. Introduction
This document describes the On-Demand Multicast Routing Protocol
(ODMRP) [14][15] developed by the Wireless Adaptive Mobility (WAM)
Laboratory [20] at University of California, Los Angeles. ODMRP
applies "on-demand" routing techniques to avoid channel overhead and
improve scalability. It uses the concept of "forwarding group," [5]
a set of nodes responsible for forwarding multicast data, to build a
forwarding mesh for each multicast group. By maintaining and using a
mesh instead of a tree, the drawbacks of multicast trees in mobile
wireless networks (e.g., intermittent connectivity, traffic
concentration, frequent tree reconfiguration, non-shortest path in a
shared tree, etc.) are avoided. A soft-state approach is taken to
maintain multicast group members. No explicit control message is
required to leave the group. We believe the reduction of
channel/storage overhead and the relaxed connectivity make ODMRP
more attractive in mobile wireless networks.
The following properties of ODMRP highlight its advantages.
* Simplicity
* Low channel and storage overhead
* Usage of up-to-date shortest routes
* Reliable construction of routes and forwarding group
* Robustness to host mobility
* Maintenance and exploitation of multiple redundant paths
* Exploitation of the broadcast nature of wireless environments
* Unicast routing capability
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2. Terminology
2.1. General Terms
This section defines terminology used in ODMRP.
node
A device that implements IP.
neighbor
Nodes that are within the radio transmission range.
forwarding group
A group of nodes participating in multicast packet forwarding.
multicast mesh
The topology defined by the link connection between forwarding
group members.
join query
The special data packet sent by multicast sources to establish
and update group memberships and routes.
join reply
The table broadcasted by each multicast receiver and forwarding
node to establish and update group membership and routes
2.2. Specification Language
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [4].
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3. Protocol Overview
3.1. Multicast Route and Mesh Creation
In ODMRP, group membership and multicast routes are established and
updated by the source on demand. Similar to on-demand unicast
routing protocols, a request phase and a reply phase comprise the
protocol. When a multicast source has packets to send but no route
and group membership is known, it floods a member advertising packet
with data payload piggybacked. This packet, called "Join Query"
(format shown in Section 4.1) is periodically broadcasted to the
entire network to refresh the membership information and update the
routes. When a node receives a Join Query packet, it stores the
source address and the unique identifier of the packet to its
"Message Cache" to detect duplicates. The upstream node address is
inserted or updated as the next node for the source node in its
"Routing Table." If the Join Query packet is not a duplicate and
the Time-To-Live value is greater than zero, appropriate fields are
updated and it is rebroadcast (operation details are illustrated
in Section 5.1.2).
When a Join Query packet reaches the multicast receiver, it creates
and broadcasts a "Join Reply" to its neighbors. When a node receives
a Join Reply, it checks if the next node address of one of the
entries matches its own address. If it does, the node realizes that
it is on the path to the source and thus is part of the forwarding
group; it sets the FG_FLAG (Forwarding Group Flag). It then
broadcasts its own Join Reply built upon matched entries. The next
node address field is filled in by extracting the information from
its routing table. This way, the Join Reply is propagated by each
forward group member until it reaches the multicast source via the
selected path. This process constructs (or updates) the routes from
sources to receivers and builds a mesh of nodes, the forwarding
group.
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+--+ +--+ +--+
|S1|-------|I1|-------|R1|
+--+\ +--+ /+--+ Join Replies of Node R1 and Node I1
\ / +----------------+ +----------------+
\ / |Sender|Next Node| |Sender|Next Node|
\ / |------+---------| |------+---------|
\ / | S1 | I1 | | S1 | S1 |
\ / |------+---------| +----------------+
+--+ \+--+/ +--+ | S2 | I2 |
|S2|-------|I2|-------|R2| +----------------+
+--+ +--+ +--+
Let us consider the above figure as an example of Join Reply
forwarding process. Nodes S1 and S2 are multicast sources, and nodes
R1 and R2 are multicast receivers. Node R2 sends its Join Reply to
both S1 and S2 via I2, and R1 sends its packet to S1 via I1 and to
S2 via I2. When receivers send their Join Replies to next hop nodes,
an intermediate node I1 sets the FG_FLAG and builds its own Join
Reply since there is a next node ID entry in the Join Reply received
from R1 that matches its ID. Note that the Join Reply built by I1
has an entry for sender S1 but not for S2 because the next node
address for S2 in the received Join Reply is not I1. In the
meanwhile, node I2 sets the FG_FLAG, constructs its own Join Reply
and sends it to its neighbors. Note that I2 broadcasts the Join
Reply only once even though it receives two Join Replies from the
receivers because the second table arrival carries no new source
information. Channel overhead is thus reduced dramatically in cases
where numerous multicast receivers share the same links to the
source.
After this group establishment and route construction process, a
source can multicast packets to receivers via selected routes and
forwarding groups. While outgoing data packets exist, the source
sends Join Query every REFRESH_INTERVAL. This Join Query and Join
Reply propagation process refreshes forwarding group and routes.
When receiving the multicast data packet, a node forwards it only
when it is not a duplicate and the setting of the FG_FLAG for the
multicast group has not expired. This procedure minimizes the
traffic overhead and prevents sending packets through stale routes.
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3.2. Reliability
The reliable transmission of Join Replies plays an important role
in establishing and refreshing multicast routes and forwarding
groups. Hence, if Join Replies are not properly delivered,
effective multicast routing cannot be achieved by ODMRP. The IEEE
802.11 MAC (Medium Access Control) protocol [8], which is the
emerging standard in wireless networks, performs reliable
transmission by retransmitting the packet if no acknowledgment is
received. However, if the packet is broadcasted, no acknowledgments
or retransmissions are sent. In ODMRP, the transmission of Join
Replies are often broadcasted to more than one upstream neighbors
since we are handling multiple sources (e.g., see the Join Reply
from node R1 in the example of Section 3.1.). In such cases, the
hop-by-hop verification of Join Reply delivery and the
retransmission cannot be handled by the MAC layer. It must be done
indirectly by ODMRP. Another option for reliable delivery is to
subdivide the Join Reply into separate sub-tables, one for each
distinct next node. In the figure of Section 3.1. for example, the
Join Reply at node R1 is split into two Join Replies, one for
neighbor I1 and the other for neighbor I2. These Join Replies are
separately unicasted using a reliable MAC protocol such as IEEE
802.11 or MACAW [3]. Since the number of neighbors is generally
limited (typically, about six neighbors in the optimum in a
multihop network [12]), the scheme still scales well to large number
of sources. This option can actually be used as backup to the
passive acknowledgment option as discussed below.
We adopt a scheme that was used in [10]. When a node transmits a
Join Reply packet to the immediate upstream node of the route, the
immediate downstream node can hear the transmission if it is
within the transmitter's radio range. Hence, the packet is used as
an "passive acknowledgment." We can utilize this passive
acknowledgment to verify the delivery of a Join Reply. Note that
the source itself must send an active acknowledgment to the
previous hop since it does not have any next hop to send a Join
Reply to unless it is also a forwarding group node for other
sources.
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Considering the case in figure of Section 3.1. again, we note that
once the nodes I1 and I2 receive the Join Reply from node R1, they
will construct and forward their own Join Replies to next hops (in
this case, sources S1 and S2). In transmitting their Join Replies,
nodes I1 and I2 may overlap with each other. If I1 and I2} are
within receiving range, they will recover because of the carrier
sense feature in CSMA (Carrier Sense Multiple Access) [13]. However,
if they are out of range, they will be unaware of the "hidden
terminal" condition of node R1, which cannot hear the (overlapped)
passive acknowledgments. Thus, a node may not hear the passive
acknowledgments of its upstream neighbor because of conflicts due
to the hidden terminal problem. It will also not hear the passive
acknowledgment if the upstream neighbor has moved away. In either
case, when no acknowledgment is received within the timeout
interval, the node retransmits the message. Note that the node may
get acknowledgments from some, but not all upstream neighbors. As
an option, the retransmission could be carried out in unicast mode,
to selected neighbors, with reduced sub-tables. If packet delivery
cannot be verified after an appropriate number of retransmissions,
the node considers the route to be invalidated. At this point, the
most likely cause of route failure is the fact that a node on the
route has failed or has moved out of range. An alternate route must
be found "on the spot." The node thus broadcasts a message to its
neighbors specifying that the next hop to a set of sources cannot
be reached. Upon receiving this packet, each neighbor builds and
unicasts the Join Reply to its next hop if it has a route to the
multicast sources. If no route is known, it simply broadcasts the
packet specifying the next hop is not available. In both cases, the
node sets its FG_FLAG. In practical implementations, this
redundancy is sufficient to establish alternate paths until a more
efficient route is established during the next refresh phase. The
FG_FLAG setting of every neighbor may create excessive redundancy,
but most of these settings will expire because only necessary
forwarding group nodes will be refreshed in the next Join Reply
propagation phase.
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3.3. Soft State
In ODMRP, no explicit control packets need to be sent to leave the
group. If a multicast source wants to leave the group, it simply
stops sending any Join Query packets since it does not have any
multicast data to send to the group. If a receiver no longer wants
to receive from a particular multicast group, it does not send the
Join Reply for that group. Nodes in the forwarding group are demoted
to non-forwarding nodes if not refreshed (no Join Replies received)
before they timeout.
3.4. Selection of Timer Values
Timer values for route refresh interval and forwarding group
timeout interval can have impacts on ODMRP performance. The
selection of these soft state timers should be adaptive to network
environment (e.g., traffic type, traffic load, mobility pattern,
mobility speed, channel capacity, etc.). When small route refresh
interval values are used, fresh route and membership information
can be obtained frequently at the expense of producing more packets
and causing network congestion. On the other hand, when large route
refresh values are selected, even though less control traffic will
be generated, nodes may not know up-to-date route and multicast
membership. Thus in highly mobile networks, using large route
refresh interval values can yield poor protocol performance. The
forwarding group timeout interval should also be carefully
selected. In networks with heavy traffic load, small values should
be used so that unnecessary nodes can timeout quickly and not
create excessive redundancy. In situations with high mobility,
however, large values should be chosen so that more alternative
paths can be provided. It is important to note that the forwarding
group timeout value must be larger (e.g., 3 to 5 times) than the
value of route refresh interval.
3.5. Unicast Capability
One of the major strengths of ODMRP is its unicast routing
capability. Not only can ODMRP coexist with any unicast routing
protocol, it can also operate very efficiently as an unicast
routing protocol. Thus, a network equipped with ODMRP does not
require a separate unicast protocol. Other ad hoc multicast routing
protocols such as AMRoute [5], CAMP [7], RBM [6], and LAM [9]
must be run on top of a unicast routing protocol. CAMP, RBM, and
LAM in particular, only work with certain underlying unicast
protocols.
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3.6. Contents of Tables
Nodes running ODMRP are required to maintain the following tables.
These tables MAY be implemented in any format, but MUST include the
fields specified in this document.
3.6.1. Routing Table
A routing table is created on demand and is maintained by each node.
An entry is inserted or updated when a non-duplicate Join Query is
received. The node stores the destination (i.e., the source of the
Join Query) and the next hop to the destination (i.e., the last
node that propagated the Join Query). The routing table provides
the next hop information when transmitting Join Replies.
3.6.2. Forwarding Group Table
When a node is a forwarding group node of the multicast group, it
maintains the group information in the forwarding group table. The
multicast group ID and the time when the node was last refreshed
are recorded.
3.6.3. Message Cache
The message cache is maintained by each node to detect duplicates.
When a node receives a new Join Query or data, it stores the source
address and the unique identifier of the packet. Note that entries
in the message cache need not be maintained permanently. Schemes
such as LRU (Least Recently Used) or FIFO (First In First Out) can
be employed to expire and remove old entries and prevent the size
of the message cache to be extensive.
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3.7. Mobility Prediction
3.7.1 Adapting the Refresh Interval via Mobility Prediction
ODMRP requires periodic flooding of Join Query to build and refresh
routes. Excessive flooding, however, is not desirable in ad hoc
networks because of bandwidth constraints. Furthermore, flooding
often causes congestion, contention, and collisions. Finding the
optimal flooding interval is critical in ODMRP performance. In
highly mobile networks where nodes are equipped with GPS [11] (e.g.,
tactical networks with tanks, ships, aircrafts, etc.), we can
efficiently adapt the REFRESH_INTERVAL to mobility patterns and
speeds by utilizing the location and movement information. We use
the location and movement information to predict the duration
of time routes will remain valid. With the predicted time of route
disconnection, Join Queries are only flooded when route breaks of
ongoing data sessions are imminent. Note that ODMRP can still
operate efficiently in networks where no such information is
available, but the protocol can be further improved if those
information can be utilized.
In our prediction method, we assume a free space propagation model,
where the received signal strength solely depends on its distance
to the transmitter. We also assume that all nodes in the network
have their clock synchronized (e.g., by using the NTP (Network Time
Protocol) [16] or the GPS clock itself). Therefore, if the motion
parameters of two neighbors (e.g., speed, direction, radio
propagation range, etc.) are known, we can determine the duration
of time these two nodes will remain connected. Assume two nodes i
and j are within the transmission range r of each other. Let
(x_{i}, y_{i}) be the coordinate of node i and (x_{j}, y_{j}) be
that of node j. Also let v_{i} and v_{j} be the speeds, and
theta_{i} and theta_{j} (0 <= theta_{i}, theta_{j} < 2 * pi) be the
moving directions of nodes i and j, respectively. Then, the
duration of time that the link between two nodes will stay
connected, D_{t}, is given by:
-(a*b + c*d) + sqrt((a^{2} + c^{2})*r^{2} - (a*d - b*c)^{2})
D_{t} = ------------------------------------------------------------
a^{2} + c^{2}
where
a = v_{i}*cos(theta_{i}) - v_{j}*cos(theta_{j}),
b = x_{i} - x_{j},
c = v_{i}*sin(theta_{i}) - v_{j}*sin(theta_{j}), and
d = y_{i} - y_{j}.
Note that when v_{i} = v_{j} and theta_{i} = theta_{j}, D_{t} is
set to infinity without applying the above equation.
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To utilize the information obtained from the prediction, extra
fields must be added into Join Query and Join Reply packets. When a
source sends Join Query, it appends its location, speed, and
direction. It sets the MIN_LET (Minimum Link Expiration Time) field
to the MAX_LET_VALUE since the source does not have any previous
hop node. The next hop neighbor, upon receiving a Join Query,
predicts the link expiration time between itself and the previous
hop using the above equation. The minimum between this value and
the MIN_LET indicated by the Join Query is included in the packet.
The rationale is that as soon as a single link on a path is
disconnected, the entire path is invalidated. The node also
overwrites the location and mobility information field written by
the previous node with its own information. When a multicast member
receives the Join Query, it calculates the predicted LET of the
last link of the path. The minimum between the last link expiration
time and the MIN_LET value specified in the Join Query is the RET
(Route Expiration Time). This RET value is enclosed in the Join
Reply and broadcasted. If a forwarding group node receives multiple
Join Replies with different RET values (i.e., lies in paths from
the same source to multiple receivers), it selects the minimum RET
among them and sends its own Join Reply with the chosen RET value
attached. When the source receives Join Replies, it selects the
minimum RET among all the Join Replies received. Then the source
can build new routes by flooding a Join Query before the minimum
RET approaches (i.e., route breaks).
In addition to the estimated RET value, other factors need to be
considered when choosing the refresh interval. If the node mobility
rate is high and the topology changes frequently, routes will
expire quickly and often. The source may propagate Join Query
excessively and this excessive flooding can cause collisions and
congestion, and clogs the network with control packets. Thus, the
MIN_REFRESH_INTERVAL should be enforced to avoid control message
overflow. On the other hand, if nodes are stationary or move slowly
and link connectivity remains unchanged for a long duration of time,
routes will hardly expire and the source will rarely send Join
Query. A few problems arise in this situation. First, if a node in
the route suddenly changes its movement direction or speed, the
predicted RET value becomes obsolete and routes will not be
reconstructed in time. Second, when a non-member node which is
located remotely to multicast members wants to join the group,
it cannot inform the new membership or receive data until a Join
Query is received. Hence, the MAX_REFRESH_INTERVAL should be set.
The selection of the MIN_REFRESH_INTERVAL and the
MAX_REFRESH_INTERVAL values should be adaptive to network
environments.
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3.7.2. Route Selection Criteria
In ODMRP, a multicast receiver selects routes based on the minimum
delay (i.e., routes taken by the first Join Query received. A
different route selection method is applied when we use the
mobility prediction. The idea is inspired by the Associativity-Based
Routing (ABR) protocol [18] which chooses associatively stable
routes. In our algorithm, instead of using the minimum delay path,
we can choose a route that is the most stable (i.e., the one that
will remain connected for the longest duration of time). To select
a route, a multicast receiver must wait for an appropriate amount of
time after receiving the first Join Query so that all possible
routes and their route qualities will be known. The receiver then
chooses the most stable route and broadcasts a Join Reply. Route
breaks will occur less often and the number of Join Query
propagation will reduce because stable routes are used.
3.7.3. Alternative Method of Prediction
Since GPS may not work properly in certain situations (e.g.,
indoor, fading, etc.), we may not always be able to accurately
predict the link expiration time for a particular link. However,
there is an alternative method to predict the LET. This method is
based on a more realistic propagation model and has been proposed
in [1] and [17]. Basically, transmission power samples are measured
periodically from packets received from a mobile's neighbor. From
this information it is possible to compute the rate of change for a
particular neighbor's transmission power level. Therefore, the time
when the transmission power level will drop below the acceptable
value (i.e., hysteresis region) can be predicted. We plan to
investigate this option in our future work.
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4. Packet and Table Formats
4.1. Join Query Packet Header
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Reserved | Time To Live | Hop Count |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Hop IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Hop X Coordinate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Hop Y Coordinate |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Hop Moving Speed | Previous Hop Moving Direction |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Minimum Link Expiration Time |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
01; ODMRP Join Query.
Reserved
Sent as 0; ignored on reception.
Time To Live
Number of hops this packet can traverse.
Hop Count
The number of hops traveled so far by this packet.
Multicast Group IP Address
The IP address of the multicast group.
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Sequence Number
The sequence number assigned by the source to uniquely
identify the packet.
Source IP Address
The IP address of the node originating the packet.
Previous Hop IP Address
The IP address of the last node that has processed this packet.
Previous Hop X Coordinate (Optional)
The x-coordinate of the last node that has processed this
packet. The information can be obtained from the GPS. This
field is required only when network hosts are GPS equipped.
Previous Hop Y Coordinate (Optional)
The y-coordinate of the last node that has processed this
packet. The information can be obtained from the GPS. This
field is required only when network hosts are GPS equipped..
Previous Hop Moving Speed (Optional)
The mobility speed of the last node that has processed this
packet. The information can be obtained from the GPS or the
node's own instruments and sensors (e.g., campus, odometer,
speed sensors, etc.). This field is required only when network
hosts are GPS equipped.
Previous Hop Moving Direction (Optional)
The moving direction of the last node that has processed this
packet. The information can be obtained from the GPS or the
node's own instruments and sensors (e.g., campus, odometer,
speed sensors, etc.). This field is required only when network
hosts are GPS equipped.
Minimum Link Expiration Time (Optional)
The minimum expiration time among the links taken by this
packet so far. This field is required only when network hosts
are GPS equipped.
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4.2. Join Reply Packet
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Count |R|F| Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Group IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Previous Hop IP Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender IP Address [1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop IP Address [1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Expiration Time [1] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| : |
| : |
| : |
| : |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sender IP Address [n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop IP Address [n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Expiration Time [n] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
02; ODMRP Join Reply.
Count
Number of (Sender IP Address, Next Hop IP Address)
combinations.
R
Acknowledgment request flag. This flag is set when active
acknowledgment packet is requested.
F
Forwarding group flag. This flag is set when the packet is
transmitted by a forwarding group node.
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Reserved
Sent as 0; ignored on reception.
Multicast Group IP address
The IP address of the multicast group.
Previous Hop IP Address
The IP address of the last node that has processed this packet.
Sequence Number
The sequence number assigned by the previous hop node to
uniquely identify the packet.
Sender IP Address [1..n]
The IP addresses of the sources of this multicast group.
Next Hop IP Address [1..n]
The IP addresses of next nodes that this packet is target to.
Route Expiration Time [1..n] (Optional)
The minimum route expiration times of this multicast group.
This field is required only when network hosts are GPS equipped.
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5. Operation
5.1. Forwarding Group Setup
5.1.1. Originating a Join Query
When a multicast source has data packets to send but no route is
known, it originates a "Join Query" packet. The Type field MUST be
set to 01. TTL MAY be set to TIME_TO_LIVE_VALUE, but SHOULD be
adjusted based on network size and network diameter. The Sequence
Number MUST be large enough to prevent wraparound ambiguity, and the
Hop Count is initially set to zero. The source puts its IP address
in the Source IP Address and Last Hop IP Address field. It appends
its location, speed, and direction into Join Query if nodes in the
network are equipped with GPS.
When location and movement information is utilized, it sets the
MIN_LET (Link Expiration Time) field to the MAX_LET_VALUE since the
source does not have any previous hop node. When the source receives
Join Replies from multicast receivers, it selects the minimum RET
(Route Expiration Time) among all the Join Replies received. Then the
source can build new routes by originating a Join Query before the
minimum RET approaches (i.e., route breaks of ongoing data sessions
are imminent).
5.1.2. Processing a Join Query
When a node receives a Join Query packet:
1. Check if it is a duplicate by comparing the (Source IP Address,
Sequence Number) combination with the entries in the message
cache. If a duplicate, then discard the packet. DONE.
2. If it is not a duplicate, insert an entry into the message cache
with the information of the received packet (i.e., sequence
number and source IP address) and insert/update the entry for
routing table (i.e., backward learning).
3. If the node is a member of the multicast group, it originates a
Join Reply packet with the RET value enclosed (see Section 5.1.4).
4. Increase the Hop Count field by 1 and decrease the TTL field by 1.
5. If the TTL field value is less than or equal to 0, then discard
the packet. DONE.
6. If the TTL field value is greater than 0, then set the node's IP
Address into Last Hop IP Address field and broadcast. DONE.
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5.1.3. Processing a Join Query When GPS is Used
When a node receives a Join Query packet:
1. Check if it is a duplicate by comparing the (Source IP Address,
Sequence Number) combination with the entries in the message
cache. If a duplicate, then discard the packet. DONE.
2. If it is not a duplicate, insert an entry into the message cache
with the information of the received packet (i.e., sequence
number and source IP address) and insert/update the entry for
routing table (i.e., backward learning).
3. Predict the duration of time the link between the node and the
upstream node will remain connected using the equation given in
Section 3.7.1.
The minimum between the newly obtained D_{t} value and the
indicated value in MIN_LET field of the Join Query is included in
the packet. The rationale is that as soon as a single link on the
path is disconnected, the entire path is invalidated. The node
also overwrites the location and mobility information field
written by the previous node with its own information.
4. If the node is a member of the multicast group, it calculates the
predicted LET of the last link of the path. The minimum between
the last link expiration time and the MIN_LET value specified in
the Join Query is the RET (Route Expiration Time).
To select a route, a multicast receiver must wait for an
appropriate amount of time after receiving the first Join Query
so that all possible routes and their RET will be known. The
receiver then chooses the most stable route (i.e., the route with
the largest RET) and originates a Join Reply packet with the RET
value enclosed (see Section 5.1.3.).
5. Increase the Hop Count field by 1 and decrease the TTL field by 1.
6. If the TTL field value is less than or equal to 0, then discard
the packet. DONE.
7. If the TTL field value is greater than 0, then set the node's IP
Address into Last Hop IP Address field and broadcast. DONE.
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5.1.4. Originating a Join Reply
A multicast receiver transmits a "Join Reply" packet after selecting
the multicast route. Each sender IP address and next hop IP address
of a multicast group are contained in the Join Reply packet. The
route expiration time is also included if the network hosts operate
with GPS.
5.1.5. Processing a Join Reply
When a Join Reply is received:
1. The node looks up the Next Hop IP Address field of the received
Join Reply entries. If no entries match the node's IP Address, do
nothing. DONE.
2. If one or more entries coincide with the node's IP Address, set
the FG_FLAG and build its own Join Reply. The next hop IP address
can be obtained from the routing table.
3. Broadcast the Join Reply packet to the neighbor nodes. DONE.
5.1.6. Processing a Join Reply When GPS is Used
When a Join Reply is received:
1. The node looks up the Next Hop IP Address field of the received
Join Reply entries. If no entries match the node's IP Address, do
nothing. DONE.
2. If one or more entries coincide with the node's IP Address, set
the FG_FLAG and build its own Join Reply. If multiple Join Replies
with different RET values are received (i.e., the node lies in
paths from the same source to multiple receivers), it selects the
minimum RET among them and attaches the chosen RET value. Next
hop IP address can be obtained from the routing table.
3. Broadcast the Join Reply packet to the neighbor nodes.
4. If the node is a source, it selects the minimum RET among all the
Join Replies received. Then the source can build new routes by
flooding a Join Query before the minimum RET approaches (i.e.,
route breaks of ongoing data sessions are imminent).
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5.2. Handling a Multicast Data Packet
Multicast sources send the data whenever they have packets to send.
Nodes relay data packets only if the packet is not a duplicate and
the setting of FG_FLAG for the multicast group has not expired.
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6. Simulation and Implementation Status
ODMRP has been implemented in both simulation and real ad hoc
network testbed. For simulation, the protocols is implemented
within the GloMoSim library [19], which is written in PARSEC[1].
As for the real system implementation, ODMRP is developed on Linux
kernel version 2.0.36, the kernel version provided by the Red Hat
Linux version 5.2. All tools and software packages that are used
in our development originate from software bundle incorporated
within the Red Hat Linux version 5.2 operating system package with
the singular exception of Lucent WaveLan IEEE 802.11 device driver.
The papers presenting our simulation results and working wireless
testbed implementation can be downloaded from the following
website:
http://www.cs.ucla.edu/NRL/wireless
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7. Protocol Applicability
7.1. Networking Context
ODMRP is best suited for mobile ad hoc wireless networks.
7.2. Protocol Characteristics and Mechanisms
* Does the protocol provide support for unidirectional links? (if so,
how?)
- No. We assume bidirectional links.
* Does the protocol require the use of tunneling? (if so, how?)
- No.
* Does the protocol require using some form of source routing? (if
so, how?)
- No.
* Does the protocol require the use of periodic messaging? (if so,
how?)
- Yes, but only when multicast sources have data packets to send.
* Does the protocol require the use of reliable or sequenced packet
delivery? (if so, how?)
- No.
* Does the protocol provide support for routing through a multi-
technology routing fabric? (if so, how?)
- No.
* Does the protocol provide support for multiple hosts per router?
(if so, how?)
- No. In this document, we assume each mobile host is combined
with a router, sharing the same IP address. It is possible,
however, to extend the protocol to handle multiple hosts per
router.
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* Does the protocol support the IP addressing architecture? (if so,
how?)
- Yes. The message contains host IP address as its identification.
* Does the protocol require link or neighbor status sensing (if so,
how?)
- No.
* Does the protocol have dependence on a central entity? (if so,
how?)
- No.
* Does the protocol function reactively? (if so, how?)
- Yes. For example, the source creates and maintains routes and
multicast group membership only when it has data packets to
send.
* Does the protocol function proactively? (if so, how?)
- No.
* Does the protocol provide loop-free routing? (if so, how?)
- Yes. By using the Message Cache, duplicate packets are detected
and packets can only go through the loop-free route.
* Does the protocol provide for sleep period operation? (if so, how?)
- TBD. The work is in progress.
* Does the protocol provide some form of security? (if so, how?)
- TBD. The work is in progress.
* Does the protocol provide support for utilizing multi-channel,
link-layer technologies? (if so, how?)
- This document assumed an arbitrary single channel link-layer
protocol. The protocol can work with any MAC and link-layer
technology. It can also support multi-channel link-layer
technology (e.g., separate channels for data, control packets,
etc.).
Lee, Su, and Gerla [Page 25]
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Acknowledgments
Authors thank Ching-Chuan Chiang and Guangyu Pei for their initial
contributions. We also send our gratitude to Sang Ho Bae who
implemented ODMRP in a real ad hoc network testbed.
References
[1] P. Agrawal, D.K. Anvekar, and B. Narendran. Optimal
Prioritization of Handovers in Mobile Cellular Networks. In
Proceedings of IEEE PIMRC'94, The Hague, Netherlands, Sep. 1994,
pp. 1393-1398.
[2] R. Bagrodia, R. Meyer, M. Takai, Y. Chen, X. Zeng, J. Martin,
and H.Y. Song. PARSEC: A Parallel Simulation Environment for
Complex Systems. IEEE Computer, vol. 31, no. 10, Oct. 1998,
pp.77-85.
[3] V. Bharghavan, A. Demers, S. Shenker, and L. Zhang. MACAW: A
Media Access Protocol for Wireless LANs. In Proceedings of ACM
SIGCOMM'94, London, UK, Sep. 1994, pp. 212-225.
[4] S. Bradner. Key words for use in RFCs to Indicate
Requirement Levels. RFC 2119, March 1997.
[5] E. Bommaiah, M. Liu, A. McAuley, and R. Talpade. AMRoute:
Adhoc Multicast Routing Protocol. Internet Draft,
draft-talpade-manet-amroute-00.txt, Aug. 1998. Work in progress.
[5] C.-C. Chiang, M. Gerla, and L. Zhang. Forwarding Group
Multicast Protocol (FGMP) for Multihop, Mobile Wireless Networks.
ACM/Baltzer Cluster Computing, vol. 1, no. 2, 1998.
[6] M.S. Corson and S.G. Batsell. A Reservation-Based Multicast
(RBM) Routing Protocol for Mobile Networks: Initial Route
Construction Phase. ACM/Baltzer Wireless Networks, vol. 1,
no. 4, Dec. 1999, pp. 427-450.
[7] J.J. Garcia-Luna-Aceves and E.L. Madruga. A Multicast Routing
Protocol for Ad-Hoc Networks. In Proceedings of IEEE
INFOCOM'99, New York, NY, Mar. 1999, pp. 784-792.
[8] IEEE Computer Society LAN MAN Standards Committee. Wireless
LAN Medium Access Protocol (MAC) and Physical Layer (PHY)
Specification. IEEE std 802.11-1997. The Institute of Electrical
and Electronics Engineers, New York, NY, 1997.
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[9] L. Ji and M.S. Corson. A Lightweight Adaptive Multicast
Algorithm. In Proceedings of IEEE GLOBECOM'98, Sydney,
Australia, Nov. 1998, pp. 1036-1042.
[10] J. Jubin and J.D. Tornow. The DARPA Packet Radio Network
Protocols. Proceedings of the IEEE, vol. 75, no. 1, Jan. 1987,
pp. 21-32.
[11] E.D. Kaplan (Editor). Understanding the GPS: Principles and
Applications, Artech House, Boston, MA, Feb. 1996.
[12] L. Kleinrock and J. Silvester. Optimum Transmission Radii for
Packet Radio Networks or Why Six is a Magic Number. In
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Birmingham, AL, Dec. 1978, pp. 4.3.2-4.3.5.
[13] L. Kleinrock and F.A. Tobagi. Packet Switching in Radio
Channels: Part I - Carrier Sense Multiple-Access Modes and Their
Throughput-Delay Characteristics. IEEE Transactions on
Communications, vol. COM-23, no. 12, Dec. 1975, pp. 1400-1416.
[14] S.-J. Lee, M. Gerla, and C.-C. Chiang. On-Demand Multicast
Routing Protocol. In Proceedings of IEEE WCNC'99, New Orleans,
LA, Sep. 1999, pp. 1298-1302.
[15] S.-J. Lee, W. Su, and M. Gerla. Ad hoc Wireless Multicast with
Mobility Prediction. In Proceedings of IEEE ICCCN'99, Boston,
MA, Oct. 1999, pp. 4-9.
[16] D.L. Mills. Internet Time Synchronization: the Network Time
Protocol. IEEE Transactions on Communications, vol. 39, no. 10,
Oct. 1991, pp. 1482-1493.
[17] B. Narendran, P. Agrawal, and D.K. Anvekar. Minimizing
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In Proceedings of IEEE GLOBECOM'94, San Francisco, CA,
Dec. 1994, pp. 1679-1685.
[18] C.-K. Toh. Associativity-Based Routing for Ad-Hoc Mobile
Networks. Wireless Personal Communications Journal, Special
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Academic Publishers, vol. 4, no. 2, Mar. 1997, pp. 103-139.
[19] UCLA Parallel Computing Laboratory and Wireless Adaptive Mobility
Laboratory. GloMoSim: A Scalable Simulation Environment for
Wireless and Wired Network Systems.
http://pcl.cs.ucla.edu/projects/domains/glomosim.html
[20] UCLA Wireless Adaptive Mobility (WAM) Laboratory.
http://www.cs.ucla.edu/NRL/wireless
Lee, Su, and Gerla [Page 27]
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Chair's Address
The Working Group can be contacted via its current chairs:
M. Scott Corson
Institute for Systems Research
University of Maryland
College Park, MD 20742
USA
Phone: +1 301 405-6630
Email: corson@isr.umd.edu
Joseph Macker
Information Technology Division
Naval Research Laboratory
Washington, DC 20375
USA
Phone: +1 202 767-2001
Email: macker@itd.nrl.navy.mil
Lee, Su, and Gerla [Page 28]
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Authors' Addresses
Questions about this document can also be directed to the authors:
Sung-Ju Lee
3771 Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 206-8589
Fax: +1 310 825-7578
Email: sjlee@cs.ucla.edu
William Su
3771 Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 206-8589
Fax: +1 310 825-7578
Email: wsu@cs.ucla.edu
Mario Gerla
3732F Boelter Hall
Computer Science Department
University of California
Los Angeles, CA 90095-1596
USA
Phone: +1 310 825-4367
Fax: +1 310 825-7578
Email: gerla@cs.ucla.edu
Lee, Su, and Gerla [Page 29]
